Re: Danish/Swedish/Norwegian/Dutch: mutual intelligibility
Swedish radio had the same type of tests.
Part of it was that you should be distinct in your pronunciation, so that the radiola speakers even of mediocre quality could reproduce an intelligible sound. I miss that today (I missed it sorely when driving forklift - noisy environment).
And for Swedish films of the 40's (pilsnerfilmer), speech could very much be described as the Danish ones above.
Clearer; crisper. Though sometimes with a very distinct Stockholm-dialect (i.e. not the standard Swedish of the times). And we had the bigger-than-life Edvard Persson with the Scanian dialect (he was very popular in Denmark too).
...men på den tiden fanns inte Wikipedia, så man fick klara sig med att förstå själv.